Capturing high-quality images using reflected ambient light involves two fundamental challenges: (i) detecting optical intensities as low as a single photon, and (ii) detecting photons over a wide dynamic range of optical intensities. To detect single photons with high fidelity (i.e., a high signal-to-noise ratio [SNR]), a detector must provide a large electrical gain in the optical-to-electrical conversion process. In essentially all semiconductor photodetectors of visible and infrared light, each incident photon creates a single photo-excited electron via the photoelectric effect. Some prior-art technologies can achieve high gain by amplifying each single photo-excited electron; for instance, image-intensifier tubes exploit microchannel plates to obtain 104-106 electrons for each incident photon. However, such large gains often give rise to saturation effects in the presence of larger optical input signals thereby limiting the dynamic range of such imagers.
Other prior-art technologies employ a much lower gain-conversion process (often limited to unity gain) and obtain measurable output signals by integrating photo-excited charge for a sufficiently long period of time. These integration strategies often involve the collection of charge on a capacitive circuit element. Thermal fluctuations give rise to noise associated with electrons that are randomly moving on to and off of integration capacitors, and for practical (i.e., non-cryogenic) operating temperatures, these imagers have sensitivities limited to values well above the single-photon limit. Moreover, the finite size of these integration capacitors establishes a constraint on the dynamic range of the signals that can be recorded during each integration cycle.
The family of semiconductor photodetectors known as avalanche photodiodes (APDs) provides optical-to-electrical gain by exploiting carrier multiplication through the impact ionization process. The APD is designed so that photo-excited charges induced in the absorption region of the device are injected into a multiplication region where they are accelerated by a large electric field. When an injected charge reaches a sufficiently high kinetic energy, it can generate an electron-hole pair through an inelastic collision with lattice atoms in a process referred to as “impact ionization.” These newly liberated carriers are then also accelerated, and the process continues to create an “avalanche” of charge until all carriers have exited the high-field multiplication region of the device.
At sufficiently large electric-field intensity, known as the “avalanche breakdown field,” there is a finite probability that the avalanche multiplication process can lead to a self-sustaining avalanche. By applying a field larger than the breakdown field, the APD is operated in a metastable state in which the injection of a single photo-excited charge can trigger the development of an easily detectable macroscopic pulse of charge in an extremely short (<1 nanosecond) avalanche build-up period. This so-called “Geiger-mode” operation can provide high-efficiency detection of single photons, and devices operated in this regime are referred to as Geiger-mode avalanche photodiodes (GmAPDs).
GmAPDs have been used in each pixel of a focal-plane array to create imagers with single-photon sensitivity. One prior-art approach uses these GmAPD arrays to count every time a photon strikes a given pixel to build an intensity image based on the number of counts per pixel in a given integration time. This approach is essentially a digital analogue to less sensitive analog imagers that operate based on capacitive integration of photo-induced current. Although this approach achieves single-photon sensitivity, its dynamic range is constrained by a significant limitation of all GmAPDs demonstrated to date. Namely, during each detection avalanche event, some fraction of the induced charges can be trapped at defects in the multiplication region. These trapped charges are released by thermionic emission from the trap sites. If the device is re-armed while trapped charges are still being released, they can initiate additional avalanches causing false detection events known as “after-pulses.” Following the detection of a single photon, therefore, GmAPDs require an appreciable “dead time” before re-arming of the device to avoid afterpulses.
The probability of after-pulse counts can be reduced by limiting the amount of charge that flows with each avalanche event, but the avalanche charge per detection event must remain large enough that it can be reliably detected by appropriate digital threshold circuits. Therefore, even in ideally designed detectors, there will always be a trade-off between after-pulse minimization and detection efficiency. The result of this trade-off is that the dead-time required between single photon detection events leads to a hard limit on the dynamic range of a single GmAPD pixel.